Mongodb connection attempt failed: SSLHandshakeFailed: SSL peer certificate validation failed: self signed certificate

I set up a mongoDB database with SSL security on an Ubuntu 16.04.5 LTS server and I have been using it for a few months. To set up SSL on mongo I followed the tutorial by Rajan Maharjan on medium.com (link). Coming back to my server after a short period of not using it, I received the following error message:

SSL peer certificate validation failed: certificate has expired

Looking at the mongo log, I found:

[PeriodicTaskRunner] Server certificate is now invalid. It expired on 2018-11-10T08:10:11.000Z

So I regenerated and re-signed all my certificates with the rootCA.key file, following the same steps as when creating certificates for the first time. I restarted mongod with:

service mongod restart

And now I have the "self signed certificate" error:

MongoDB shell version v4.0.2
connecting to: mongodb://IP:port/
2018-11-21T13:11:10.584+0000 E NETWORK  [js] SSL peer certificate     
validation failed: self signed certificate
2018-11-21T13:11:10.584+0000 E QUERY    [js] Error: 
couldn't connect to server IP:port, connection attempt failed: 
SSLHandshakeFailed: SSL peer certificate validation failed: self signed certificate :
connect@src/mongo/shell/mongo.js:257:13
@(connect):1:6
exception: connect failed

My connection string is the following:

mongo --ssl --sslCAFile /PATH/TO/rootCA.pem --sslPEMKeyFile /PATH/TO/mongodb.pem --host IP:port

I have not found any resources on re-signing certificates for mongoDB. Any help would be much appreciated.

EDIT

/etc/mongod.conf :

# mongod.conf

# for documentation of all options, see:
#   http://docs.mongodb.org/manual/reference/configuration-options/

# Where and how to store data.
storage:
  dbPath: /var/lib/mongodb
  journal:
    enabled: true
#  engine:
#  mmapv1:
#  wiredTiger:

# where to write logging data.
systemLog:
  destination: file
  logAppend: true
  path: /var/log/mongodb/mongod.log

# network interfaces
net:
  port: PORT
  bindIp: IP
  ssl:
    mode: requireSSL
    PEMKeyFile: /etc/ssl/mongodb.pem
    CAFile: /etc/ssl/rootCA.pem

# how the process runs
 processManagement:
  timeZoneInfo: /usr/share/zoneinfo

#security:
security:
   authorization: "enabled"

#operationProfiling:

#replication:

#sharding:

## Enterprise-Only Options:

#auditLog:

#snmp:

Solution

I have managed to get it to work again by following these steps (which, for some reason, did not work before):

Remove mongodb.* files from /etc/ssl/

regenerate certificates with the commands

openssl genrsa -out mongodb.key 2048
openssl req -new -key mongodb.key -out mongodb.csr

Server address in the field Common Name (eg, YOUR name) []

Generate the certificate

openssl x509 -req -in mongodb.csr -CA rootCA.pem -CAkey /PATH/TO/rootCA.key -CAcreateserial -out mongodb.crt -days 500 -sha256

cat mongodb.key mongodb.crt > mongodb.pem

chmod 666 mongodb.pem

service mongod restart

cat /var/log/mongodb/mongod.log (*to check status*)

And to launch the mongo shell

mongo --ssl --sslCAFile /PATH/TO/rootCA.pem --sslPEMKeyFile /PATH/TO/mongodb.pem --host IP:PORT

Thank you @AniketMaithani for trying to help me solve this issue.